www.nature.com/scientificreports OPEN Coincidence of photic zone euxinia and impoverishment of arthropods in the aftermath of the Frasnian- Famennian biotic crisis Krzysztof Broda1*, Leszek Marynowski2, Michał Rakociński1 & Michał Zatoń1 The lowermost Famennian deposits of the Kowala quarry (Holy Cross Mountains, Poland) are becoming famous for their rich fossil content such as their abundant phosphatized arthropod remains (mostly thylacocephalans). Here, for the frst time, palaeontological and geochemical data were integrated to document abundance and diversity patterns in the context of palaeoenvironmental changes. During deposition, the generally oxic to suboxic conditions were interrupted at least twice by the onset of photic zone euxinia (PZE). Previously, PZE was considered as essential in preserving phosphatised fossils from, e.g., the famous Gogo Formation, Australia. Here, we show, however, that during PZE, the abundance of arthropods drastically dropped. The phosphorous content during PZE was also very low in comparison to that from oxic-suboxic intervals where arthropods are the most abundant. As phosphorous is essential for phosphatisation but also tends to fux of the sediment during bottom water anoxia, we propose that the PZE in such a case does not promote the fossilisation of the arthropods but instead leads to their impoverishment and non-preservation. Thus, the PZE conditions with anoxic bottom waters cannot be presumed as universal for exceptional fossil preservation by phosphatisation, and caution must be paid when interpreting the fossil abundance on the background of redox conditions. 1 Euxinic conditions in aquatic environments are defned as the presence of H2S and absence of oxygen . If such conditions occur at the chemocline in the water column, where light is available, they are defned as photic zone euxinia (PZE). In these oxygen-free and hydrogen sulphide-rich environments, some bacteria (e.g., purple sul- phur bacteria or green sulphur bacteria) can thrive and photosynthesise, thereby playing a crucial role in the sulphur cycle in stratifed basins2. In ancient sedimentary rocks, anoxygenic photosynthesis and thus the presence of PZE are best evidenced by the presence of specifc ‘molecular fossils’ (biomarkers), which are generated by anaerobic and photosynthetic bacteria3–5. PZE was an important environmental factor during the major extinc- tion events in the Earth’s history, especially during oceanic anoxic events such as the end-Ordovician6, latest Devonian5 and end-Permian7. Te PZE has also been proposed as a factor leading to mass mortality events of fsh preserved in the Cretaceous Santana Formation8. Recently, researchers have emphasised that PZE played a crucial role in the exceptional preservation of phosphatic fossils in the famous Upper Devonian conservation deposits of the Gogo Formation in Australia2. In this paper, we document the recurrent PZE conditions during sedimentation of the fossiliferous lower Famennian (Upper Devonian) deposits at the Kowala quarry (Holy Cross Mountains, Poland). Because of the great abundance and excellent preservation of phosphatic fossils of thylacocephalan, phyllocarid and angus- tidontid crustaceans9–12, articulated coelacanth fish13, conulariids14, coprolites15, as well as carbonaceous non-biomineralised algae16, these deposits were coined as the Kowala Lagerstätte9. Interestingly, these fossilif- erous deposits originated during the earliest Famennian, so in the afermath of the famous Frasnian-Famennian (F-F) biotic crisis, during which many marine groups sufered extinction and, especially, reef ecosystems col- lapsed17,18. Although intense volcanism has been considered as the most probable ultimate cause of the F-F event (e.g.19,20), the hypotheses concerning the exact proximate kill mechanism of the crisis difer21. However, the 1Department of Palaeontology and Stratigraphy, University of Silesia in Katowice, Faculty of Earth Sciences, Będzińska 60, 41-205, Sosnowiec, Poland. 2Department of Geochemistry, Mineralogy and Petrography, University of Silesia in Katowice, Faculty of Earth Sciences, Będzińska 60, 41-205, Sosnowiec, Poland. *email: [email protected] SCIENTIFIC REPORTS | (2019) 9:16996 | https://doi.org/10.1038/s41598-019-52784-4 1 www.nature.com/scientificreports/ www.nature.com/scientificreports Figure 1. Locality of the studied section. (a) Location of the Kowala quarry on the background of the geology of the Holy Cross Mountains; (b) Te Kowala quarry with an exact position of the investigated section indicated (Google Maps, 2019). common association of black “Kellwasser” facies near the F/F boundary22 supports the event might have been related to the transgression-related anoxia23,24. Combining palaeontology and geochemistry, we show that afer the F-F biotic crisis, the PZE still occurred intermittently during the early Famennian in the Late Devonian shelf basin of the southern margin of the Laurussia continent (present day Holy Cross Mountains, Poland). We also show that PZE did not lead to mass mortality events of arthropods, which abundantly inhabited the basin and, importantly, have not promoted their fossilisation as was considered previously. Our study shows that euxinic conditions should not be used as univer- sal explanation for exceptional fossil preservation. Instead, a strong caution must be paid when interpreting the taphonomy of phosphatised fossils and fuctuations of their abundance in the rock record. Geological Background Te active Kowala quarry is located in the southern limb of the Gałęzice-Bolechowice syncline in the southern part of the Kielce region of the Holy Cross Mountains (Fig. 1). During the Devonian, the Holy Cross Mountains area was part of a carbonate shelf that extended along the southern margin of the continent of Laurussia near the equator25–29. Te Famennian sediments of the Kowala quarry were deposited in the intrashelf Chęciny-Zbrza basin28,30,31. Te lower Famennian deposits investigated in this paper are 21 m thick and crop out in a trench located in the north-central part of the quarry (N50°47′43,476′′, E20°33′53,568′′, Fig. 1). Te section comprises monotonous deposits of thin-bedded, dark, carbonaceous shales and thin-bedded, grey, micritic limestones. Te investigated interval is confned to lithologic unit H-4 of Racki & Szulczewski32, which stratigraphically encom- passes the Late triangularis through Early marginifera conodont zones32. Earlier, the lower Famennian interval investigated in this paper was assumed as being confned to the crepida conodont Zone9,15, on the basis of lith- ological similarities and its position relative to the neighbouring trench section investigated by Marynowski et al.33. However, the new conodont dating showed that our section is slightly older and represents the Palmatolepis minuta minuta Zone (according to the latest conodont zonation of Spalletta et al.34), which corresponds to the previously established Late triangularis conodont Zone of Ziegler & Sandberg35. The conodont assemblage includes: Palmatolepis protorhomboidea, Pa. delicatula delicatula, Pa. superlobata, Polygnathus brevilaminus, Po. procerus, Icriodus alternatus alternatus, and Ic. deformatus deformatus. Te depositional environment is generally interpreted as deep shelf, below storm wave-base, but at least epi- sodically within the limits of the photic zone33,36–38. Material and Methods Collection of fossils. We collected 2029 specimens in total directly from the investigated section. At least 32 specimens were collected from each of the 34 investigated shale or marly shale beds, regardless of the preservation state or systematic position of the specimens. Te specimens were not retrieved from limestones as they are much harder to work with because they are very likely to be damaged while splitting the rock. For the purposes of this study, fossilised regurgitates, coprolites, and trace fossils were excluded, as they repre- sent the signs of animal activity, and not the animal itself. In some cases, the total abundance of some body fossils within each bed was difcult to establish because their skeletons tend to disarticulate post-mortem (crinoids, coelacanth fsh) or were extremely rare (angustidontid arthropods); these fossils were marked on diagrams but not evaluated quantitatively. Finally, we included 1840 of the 2029 specimens collected in situ in our quantitative palaeoecological analyses. Since during the feldwork, obtaining the same number of specimens for each bed was not always achievable and some of the collected specimens were excluded from the study, the general number of each fossil type in each bed was converted to a percentage contribution. Such a calculation allowed for comparisons with all the needed proportions being saved, despite the diferences in total number of specimens between the studied beds. Total organic carbon (TOC) and total sulphur (TS) content. For total carbon (TC), total inorganic carbon (TIC) and total sulphur (TS) measurements, an ELTRA CS-500 IR-analyser was used (at Faculty of Earth Sciences, University of Silesia in Katowice, Poland). TOC was calculated as the diference between TC and TIC. SCIENTIFIC REPORTS | (2019) 9:16996 | https://doi.org/10.1038/s41598-019-52784-4 2 www.nature.com/scientificreports/ www.nature.com/scientificreports Figure 2. Percentage contribution of the investigated lower Famennian body fossils from the Kowala quarry. Only specimens found in situ are included. Note that brachiopods (linguloid + rhynchonellid) dominate the assemblage. ELTRA